[AIRFLOW-4904] Retrieve test config file from $AIRFLOW_TEST_CONFIG (#5540)
[AIRFLOW-4904] Retrieve test config file from $AIRFLOW_TEST_CONFIG (#5540)

It defaults to the same location but this allows the location to be changed.

We may remove this test config file entirely in the future, but for now this lets the
location of this file be controlled.
airbnb/airflow,Acehaidrey/incubator-airflow,Fokko/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,spektom/incubator-airflow,Fokko/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,spektom/incubator-airflow,DinoCow/airflow,Fokko/incubator-airflow,wileeam/airflow,DinoCow/airflow,apache/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,wooga/airflow,Fokko/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,lyft/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,mrkm4ntr/incubator-airflow,nathanielvarona/airflow,apache/airflow,airbnb/airflow,spektom/incubator-airflow,wooga/airflow,bolkedebruin/airflow,sekikn/incubator-airflow,wooga/airflow,sekikn/incubator-airflow,apache/airflow,mistercrunch/airflow,spektom/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,mtagle/airflow,wileeam/airflow,mrkm4ntr/incubator-airflow,wileeam/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/airflow,apache/incubator-airflow,wooga/airflow,bolkedebruin/airflow,mistercrunch/airflow,apache/incubator-airflow,wileeam/airflow,mtagle/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,DinoCow/airflow,apache/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,bolkedebruin/airflow